The invention has applications in particular in cobotics in which robots (or cobots) work in direct and intuitive collaboration with one or more operators with whom they share working space.
Progress in the field of robotics makes it possible to envisage having an element that is manipulated simultaneously by an operator and a robot (this may be referred to as co-manipulation or co-working) in such a manner as to associate the operator's intelligence and skill with the robot's strength and accuracy.
Robots are thus known that comprise a base having mounted thereon a movable segment associated with motor-drive means that are connected to a control unit. The movable segment has an end portion provided with a member for holding an element that is to be manipulated and a handle for enabling an operator to manipulate said end portion. The control unit is connected to force sensors fastened between said end portion and the operator's handle, and between said end portion and the element that is to be manipulated. The control unit is arranged to control the motor-drive means so as to amplify the force exerted by the operator by a predetermined factor.
The operator then guides the end portion of the movable segment and thus of the element that is attached thereto, while adapting to the constraints and hazards of the operation that is to be performed, leaving it to the robot to exert the force that needs to be applied to the element that is to be manipulated.
The development of those techniques comes up against the high level of inertia in robots, in particular industrial robots, that makes them relatively uncomfortable or even dangerous for an operator to use. This residual control inertia is due firstly to the inertia of the robot itself, and secondly to the lag introduced by the control system and the transmission of the force exerted on the element by the robot. The residual inertia prevents the operator from moving the movable segment easily. In addition, when the manipulated element is a blade, force amplification facilitates cutting operations, but also makes the blade even more dangerous, should the blade separate suddenly from the material it is cutting, since it is then moved with the predominant inertia of the robot multiplied by the force amplification gain.
In order to improve the performance of such robotic devices, the control unit is arranged to control the motor-drive devices in impedance as a function of data coming from sensors so as to compensate for friction internal to the structure of the device. Nevertheless, that does not make it possible to reduce significantly the inertia that is perceived by the operator. This limitation results mainly from the presence of a mechanical system between the motor-drive means and the force sensors, which mechanical system possesses modes of vibration that are not modeled at low frequency (at about ten hertz approximately). In addition, if the servo-control gains are increased in order to reduce the inertia perceivable by an operator, instabilities are generated by the motor-drive means when the operator takes a firm grip on the handle. Mechanical filters have been mounted in association with the sensors in order to damp said vibration, but such filters, made of flexible material, become flattened when forces increase and they then no longer perform their filtering function.
Attempts have also been made to remedy those drawbacks by using robots specially designed to limit inertia, e.g. by means of a lightweight structure. Nevertheless, such robots present drawbacks of being less rigid and less robust than industrial robots, while also being more expensive. Robots presenting low inertia and little friction are also generally of limited capacity in terms of producing force.
Various sensor arrangements have also been envisaged, but without achieving significant improvement.